It's like anything in life. For example, if you work at a company you start in a small specified department working on something generally with a smaller scope. But as you get promoted, you learn more and more about the workings of the entire company and how everything fits together. While you can speak intelligently about any department in the company it is likely you'll be able to speak most comfortably about the department which you started in because you lived it and experienced it. There's little things you can only learn from having experience in that specific spot.
So IMO, while Pochettino or any manager can coach a whole team and pick a player at any position, he's also spent his whole footballing life playing defender while he's been coaching for a much shorter period. Because of that experience he probably just has a slightly keener eye for a defender because he's amassed more knowledge on being a defender. It's not to say he's weak in other categories just that he has more first hand experience as a defender than as a striker. Everyone in any walk of life has a little bit of a better understanding of something they've done at high level rather than just something they've learned not through first hand experience.
